1. Field of the Invention
The present invention relates to an image forming apparatus having a fixing unit by which an unfixed toner image formed on a recording material is fixed onto the recording material by heat.
2. Description of the Related Art
There are known to be various types of recording material heating devices in image forming apparatuses, such as a heat-roller type and a film-heating type. All of such heating devices have a heating element, and temperature management is performed by controlling the supply of power to the heating element such that the apparatus temperature is maintained at a predetermined temperature (e.g., a predetermined image fixing temperature). Among conventional heating devices, film heating-type heating devices are particularly effective and practical (Japanese Patent Laid-Open No. 4-44075).
A film heating type of heating device can use a thin film or heating element that rises in temperature quickly, having a low heat capacity, thus enabling the conservation of energy and the shortening of the wait time (quick starting). Also, in recent years, there has been a proposal for a heating device configured so as to suppress uneven melting caused by unevenness of the recording material, by providing the heating film with an elastic layer (Japanese Patent Laid-Open No. 11-15303). In the temperature control of a film heating type of heating device, the output of a thermistor provided on the heating element is subjected to A/D conversion and then input to the CPU, and in accordance with the result of a comparison between the detected temperature and a target temperature, the supply of power to the heating element is controlled through PID control based on a control table that has been determined in advance. Note that PID control refers to control in which control values are determined by combining proportional control (hereinafter referred to as “P control”), integral control (hereinafter referred to as “I control”), and derivative control (hereinafter referred to as “D control”) in accordance with output values from a control target. Also, the control of the supply of power to the heating element is performed by switching the AC voltage on and off using a controlling semiconductor switch (hereinafter referred to as a “triac”), and wave number control or phase control is used in the power supply control system.
Here, wave number control refers to control for using a certain number of waves of an input AC voltage as a predetermined cycle and performing on/off switching in units of one halfwave in the predetermined cycle, and is a system of controlling the power supply rate using the on/off duty cycle in the predetermined cycle. On the other hand, phase control is a system of controlling the phase angle in one wave of the AC input voltage. A characteristic of wave number control is that harmonic current is low and flicker noise is high, and a characteristic of phase control is that flicker noise is low and harmonic current is high. In particular, in recent years, wave number control has often been employed instead of phase control in the case of using a 200 V based commercial power supply, in order to reduce harmonic current. For this reason, there has also been a proposal for an apparatus configured so as to switch between wave number control and phase control depending on the AC input voltage, such as depending on whether the voltage is 200 V or 100 V (Japanese Patent Laid-Open No. 10-333490). There has also been a proposal for combining phase control and wave number control and using phase control in at least one halfwave in wave number control so as to perform more detailed control in which harmonic current is reduced more than in the case of performing only phase control, and the power supply rate update cycle is shorter than that in the case of performing only wave number control (Japanese Patent Laid-Open No. 2003-123941).
Incidentally, with the above-described film heating-type heating devices, and particularly with an apparatus in which the heating film is provided with an elastic layer, there are cases where the heated state of the recording material becomes unstable depending on the entry of the recording material into the heating nip portion. If the recording material enters while the temperature is stable, heat is rapidly absorbed immediately after the recording material enters the heating nip portion, and the temperature of the heating film rapidly decreases. Thereafter, overshooting occurs when the temperature rapidly rises, and thus a large temperature fluctuation occurs in the heating nip portion. In order to avoid this phenomenon, a method has been proposed in which the amount of power supplied to the heating element is corrected before the temperature fluctuation occurs due to the entry of a recording material (Japanese Patent Laid-Open No. 2004-078181). When the temperature of the heating film rapidly decreases along with the entry of the recording material into the heating nip portion, the temperature remains low when this portion again comes into contact with the recording material after the heating film has rotated one time. In other words, the temperature of the heating film decreases in the portion corresponding to the second rotation of the heating film on the recording material, thus resulting in the phenomenon in which image glossiness decreases. On the other hand, the large decrease in the temperature of the heating film due to the entry of the recording material occurs only momentarily, immediately after the heating state has rapidly changed due to the entry of the recording material. Due to performing PID control, the heating state immediately stabilizes to a certain extent, and the decrease in temperature is resolved. Meanwhile, even in the portion corresponding to the second rotation of the heating film on the recording material, image glossiness decreases only in the portion corresponding to the leading edge in the second rotation. However, there is a large difference in image glossiness between the portion at the leading edge of the second rotation of the heating film and the portion at the trailing edge of the first rotation. For this reason, there are cases where the difference in glossiness appears as a prominent change at the border between these portions. This phenomenon is particularly significant when glossy paper has been fed. In order to suppress this change in glossiness, it is necessary to perform more detailed control of the above-described power correction so as to make the glossiness match at the junction between the first rotation and the second rotation. In other words, it is necessary to compensate for the decrease in the temperature of the heating film in the portion corresponding to the leading edge of the second rotation such that even if heat is absorbed at the leading edge of the first rotation, the temperature is the same at the leading edge of the second rotation and the trailing edge of the first rotation.
The mechanism for compensating for a temperature decrease using power correction is as follows. First, the temperature of the heating film surface decreases due to the entry of a recording material. If power correction is not performed, the temperature in this portion remains low, and a change in glossiness appears after one rotation of the heating film as described above. In contrast, assume that power correction for forcibly inputting a predetermined power in anticipation of the entry of the recording material has been performed. In this case, although the temperature of the heating film surface decreases, the power (i.e., thermal energy) forcibly input within one rotation is transmitted to the heating film surface. The amount of decrease in temperature is thus canceled out, and the predetermined temperature is restored when the leading edge in the second rotation of the heating film, which corresponds to the recording material entry portion of the heating film, again comes into contact with the recording material. As can be understood from this mechanism, the portion in which the heat generated by the power correction heats the inner surface of the heating film needs to substantially match the portion in which the temperature decreased due to the entry of the recording material. Such a case requires stricter precision than the case of simply stabilizing temperature control. With a recording material such as glossy paper in particular, the glossiness is very highly sensitive to temperature, and a slight temperature difference appears as a glossiness difference (i.e., a change in glossiness), and therefore the range in which the surface temperature is to be controlled is very narrow.
In order to cause the trailing edge of the first rotation and the leading edge of the second rotation of the heating film to have the same temperature, it is necessary to perform power correction for accurately compensating for the temperature decrease at the leading edge in the second rotation. Specifically, high precision is required for not only the amount of power, but also the time at which power correction is performed. This is because change in glossiness occurs in a delta function manner. Accordingly, compensating for the temperature reduction so as to resolve this problem requires the power to be compensated for at a precise time in a delta function manner with respect to the time at which change in glossiness occurs. If the power correction time deviates even slightly from the appropriate correction time, it is not possible to sufficiently compensate for the temperature decrease due to insufficient power, or hot offsetting or the like occurs due to excessive power input. In other words, if the time at which power correction is started deviates even slightly, the effect of the power correction fades. However, with an apparatus employing wave number control, it is not possible to perform correction when power correction is to be performed with respect to the entry of a recording material. Accordingly, wave number control has the issue that a temperature fluctuation due to the entry of a recording material cannot be sufficiently suppressed. This is due to the fact that the update frequency is low since the power supply rate update cycle in wave number control is a unit of several halfwaves, and as a result, there are almost no cases in which the update time matches the power correction time.
FIG. 15 is a timing chart showing the update cycle and update timing for the power supply rate in wave number control and phase control, and the timing of recording material entry and power correction. In this example, the power supply rate update cycle in wave number control is assumed to be 20 halfwaves. The graph entitled “UPDATE CYCLE IN WAVE NUMBER CONTROL” shows the power supply rate update timing in wave number control. The graph entitled “UPDATE CYCLE IN PHASE CONTROL” shows the power supply rate update timing in phase control. Power correction is executed at time C. The recording material enters the heating nip portion at time D. In the example shown in FIG. 15, power correction is started 150 msec before the time when the recording material enters the heating nip portion, and power correction ends when 50 msec has elapsed after the time when the recording material entered the heating nip portion. The power supply rate update cycle is long in wave number control. For this reason, there is a large difference (deviation) between the appropriate correction time and the time when correction is actually performed. Since the power supply rate is controlled in intervals of 20 halfwaves in the example shown in FIG. 15, a deviation (delay) of up to 200 msec (in the case of 50 Hz) occurs from when the power correction start instruction is issued until correction is actually executed. In this case, the power correction period is from 150 msec before recording material entry until 50 msec after entry, which is 200 msec in total. For this reason, in the case where the deviation has reached the maximum value, power correction is started at the power correction end time. In other words, a power correction end instruction is actually issued at the same time as the start of power correction, and therefore power correction is not performed.
In the above-described example, the power supply rate is updated once the correction start instruction has been issued. For this reason, the timing deviation is always in the direction of delay of the execution of correction. In contrast, the power correction start time is known in advance. For this reason, based on the assumption of deviation, the maximum amount of deviation can be somewhat reduced by performing correction upon the arrival of the power supply rate update time that is closest to the power correction start time. However, even in this case, the amount of deviation can be up to ±100 msec from the appropriate power correction time.
FIGS. 16A to 16C are graphs showing the state of the heating film surface temperature in cases where the power correction time and the power supply rate update time deviate from each other. In the graphs of FIG. 16A to 16C, the horizontal axis indicates time (msec), and the vertical axis indicates the heating film surface temperature (° C.). FIG. 16A shows the case where power correction is performed at the appropriate time, FIG. 16B shows the case where the deviated start of power correction is before the appropriate time, and FIG. 16C shows the case where the deviated start of power correction is after the appropriate time. The heating film temperature decreases due to the recording material having entered the heating nip portion. However, in FIG. 16A, the difference in the heating film surface temperature before and after the entry of the recording material into the heating nip portion falls within approximately Δ2 deg. In contrast, in FIG. 16B, the surface temperature rises a large amount before the entry into the heating nip portion. For this reason, the difference in the heating film surface temperature before and after the entry into the heating nip portion is Δ8 deg. Also, in FIG. 16C, the heating film temperature decreases a large amount due to the recording material having entered the heating nip portion. For this reason, the difference in the heating film surface temperature is approximately Δ8 deg, as expected.
As is clear in FIG. 16B, in the case where power correction is performed at a deviated time, if correction is performed before the appropriate time, the heating nip portion temperature rises excessively, and overheating occurs. If a recording material holding a toner image enters in this state, the toner melts excessively, and hot offsetting will occurs. Also, since a large amount of power is supplied before the appropriate time, the heating film temperatures rises excessively in the period up to when the recording material enters, and the glossiness of the recording material rises in the portion corresponding to the trailing edge of the first rotation of the film. Accordingly, horizontal band shaped glossiness unevenness occurs such that the change between the trailing edge of the first rotation and the leading edge of the second rotation is emphasized. On the other hand, if correction is performed after the appropriate time as shown in FIG. 16C, it is not possible to compensate for the decrease in heat due to recording material entry, and the temperature decreases by a large amount. In this case, the glossiness decreases excessively in the portion corresponding to the second rotation of the heating film. Specifically, the change between the trailing edge of the first rotation and the leading edge of the second rotation becomes prominent, and glossiness unevenness occurs. In order to address this issue, it is possible to shorten the power supply rate update cycle, but in this case, it is not possible to perform detailed setting of the power supply rate since the number of waves in the update cycle decreases, thus bringing about an obstacle in temperature control.
Incidentally, timing deviation occurs in the case of phase control as well. Although the maximum value of deviation is 1 full wave, which is 20 msec (in the case of 50 Hz), even this extent of deviation cannot be said to have no influence. However, as a result of examination, the inventors found that at this extent of deviation, the glossiness unevenness manages to fall within an allowable range. To put it the other way around, unless phase control is used, timing deviation cannot be suppressed to an allowable level. However, since phase control has the issue of harmonic current, there are necessarily cases where phase control cannot be employed, as described above. In particular, in Europe where the commercial alternating-current power supply voltage is 200 V, regulations regarding harmonic current are strict, and it is necessary to use wave number control instead of phase control.
Also, with the wave number control disclosed in Japanese Patent Laid-Open No. 2003-123941, the power supply rate update cycle can be shortened in control performed using phase control in at least one halfwave in the power supply rate update cycle, thus having the effect of somewhat of an improvement regarding this problem, that is to say, the problem of deviation of the power correction timing. However, when the number of waves in the update cycle decreases as a result of shortening the power supply rate update cycle, the number of waves to perform phase control relatively increases, and therefore harmonic current increases. Also, as described above, deviation of the power correction timing manages to fall within the allowable range if phase control is used in all of the waveforms, and therefore there is a limit to the suppression of deviation of the power correction timing even in the case of using waveforms combining phase control and wave number control.